578 MR J. A. BROUN ON THE 
29. I believe then that I have shown, in opposition to Dr Wotr’s conclusion, 
that there was a maximum in 1797; in opposition to Dr Lamont, that GILPin’s 
observations are in all probability trustworthy; that there was probably a period 
of small variation in. the amplitude of the diurnal oscillation; which, should it 
recur, may aid in the discovery of the cause of the variations of sun-spot and 
auroral frequency, as well as of the magnetic disturbances; and that the mean 
durations of the period is about 10°45 years.* 
30. Maximum of 1776.—It will have been seen (Art. 22) that the long period 
of 42 years gives 17762 as the epoch of maximum before 1787:°2. Dr Wo.F 
finds from his spot numbers that 1775°8 was the epoch of minimum, and that 
1779 was the year of maximum; this also agrees with Professor Loomis’s 
numbers of aurore. We have, fortunately, two series of magnetical observa- 
tions, which may aid in deciding whether 1776 was a year of maximum or of 
minimum. I have already referred to VAN SWINDEN’s result (Art. 2); the fol- 
lowing are the numbers of days of disturbance observed by him in each year, 
as well as the numbers of irregular days on which the north end of the needle 
attained its most westerly position before noon or after 4 P.M.t 
Wear! : Numbers of Days of f 
Disturbance. Of Irregular Maximum. 
1771 c : : 8 : : : 104 
1772 . : : 20 : : : 71 
aire yee) : é , 33 ‘ : : 83 
1774 5a : : 35 é : 111 
1775 : : : 27 : : : 120 
1776 ; : ; 16 : : : 137 
1777 : 3 é 34 : : : 113 
1778 5 : ; Le? ‘ ; : 108 
ers) : : : 21 : : ; 81 
1780 . : ‘ Wi . . . 91 
1781 : : : 14 
It has been stated that VAN SwINDEN does not seem to have had any exact 
limit to define his days of disturbance, and all that can be deduced from the 
first column of numbers is, that the maximum appears to have occurred 
period of the Zurich astronomer. It should also be remarked that the longest interval for two successive 
periods given by Dr Wotr before 1787 is 26 years; since 1818, the longest is 23 years; while from 
1787 to 1818 gives 31 years, which, for any other two periods, is an interval unknown in Dr Wotr'’s 
sun-spot history. It is obvious that if 10°4 years be near the mean duration, the last result of 11°8 
years, obtained on the supposition that there was no maximum in 1797, will go on diminishing, passing 
through the mean of 11-1 years about 1960. 
* Tt will be remarked that this is very nearly the period obtained in 1862 by Dr Lamont (Art. 6), 
I would therefore repeat that his result was founded on two hypotheses :—1st, That the length of 
the period should always have been within the limits observed since 1818. 2d, That Giupin’s obser- 
vations and the sun-spot numbers of Dr WoxF (which did not satisfy the first hypothesis) were worth- 
less. These hypotheses seem inadmissible, and Dr Wotr’s result has been in consequence very generally 
accepted. The whole discussion induces me to believe that a maximum occurred near 1797, and the 
only point on which any doubt can remain is as to its magnitude,—whether it was really so small as” 
all the observations indicate. 
+ “ Analogie de 1’Elect. et du Mag.” t. iii. pp. 85, 129. 

